1. Field of the Invention
The present invention relates to an ignition coil. More particularly, the present invention relates to an ignition coil of a stick type mounted directly on an ignition plug hole of an engine.
2. Description of the Related Art
A sectional view in a direction perpendicular to the axis of an ignition coil 100 near a laminated core 101 of an ignition coil 100 is shown in FIG. 8. As shown schematically, the laminated core 101 is rod shaped. The laminated core 101 is made up of a plurality of strip-shaped thin silicon steel plates 102 stacked in the radial direction. Around the outer circumferential surface of the laminated core 101, a tape 103 made of poly ethylene terephthalate (PET) is wound. Outside the outer circumference of the tape 103, a cylindrical secondary spool 104 is arranged coaxially with the laminated core 101. A gap 105 is defined between the inner circumferential surface of the secondary spool 104 and the outer circumferential surface of the tape 103. A secondary coil 106 is wound around the outer circumferential surface of the secondary spool 104. Each member, described above, is contained in a housing (not shown), which is the outer shell of the ignition coil 100.
An epoxy resin is injected into the housing. The gap between the individual members in the housing is filled with the epoxy resin, which hardens therein. The epoxy resin ensures insulation between the individual members. Moreover, the epoxy resin fixes each member. The gap 105 also is filled with an epoxy resin 107a. FIG. 9 shows the sectional view taken along the line I—I in FIG. 8. As shown schematically, the gap between the secondary coil 106 and the outer circumferential surface of the secondary spool 104 also is filled with an epoxy resin 107b. 
The linear expansion coefficients differ between the epoxy resin, the secondary coil 106 and the secondary spool 104. At a low temperature, the linear expansion coefficient of the secondary coil 106 is lower than that of the secondary spool 104 and that of the epoxy resin. Because of this, the secondary spool 104 and the epoxy resin 107a shown in FIG. 9 tend to contract and deform in the direction toward the center of the secondary spool 104, that is, in a state in which the radii thereof are reduced. In contrast to this, the secondary coil 106 hardly deforms. However, the secondary winding 106 and the secondary spool 104 are linked to each other via the epoxy resin 107b present in the gap. Therefore, even though the secondary spool 104 and the epoxy resin 107a, which tend to contract and deform in the direction toward the center of the secondary spool 104, that is, in a state in which the radii thereof are reduced, are prevented from doing so, by the secondary coil 106, from the outer circumferential side. In other words, a thermal stress is applied from the outer circumferential side to the members arranged within the inner circumferential side of the secondary coil 106. To be specific, thermal stress 109 acts in the circumferential direction as shown by the arrow in FIG. 8
On the other hand, the laminated core 101 is made up of the plurality of stacked silicon steel plates 102. Each of the stacked silicon steel plates 102 becomes warped and deformed by a small amount because of the thermal stress due to cooling load of an engine. Therefore, if the laminated core 101 is in contact with the epoxy resin 107a in a bare state, the laminated core 101 is deformed into an elliptic shape because of the warpage and deformation of the silicon steel plate 102, as shown exaggeratedly by a dotted line 110 in FIG. 8. Due to the elliptic deformation of the laminated core 101, a thermal stress 111 is applied to the epoxy resin 107a in the direction of the longitudinal axis of the ellipse, as shown by the arrow in FIG. 8. Because of the combined effect of the thermal stress 111 in the direction of the longitudinal axis of the ellipse and the thermal stress 109 in the circumferential direction, a large thermal stress is applied to the epoxy resin 107a as a result.
Moreover, if the laminated core 101 is in contact with the epoxy resin 107a in a bare state, there is the possibility that a crack may occur starting from a pointed corner portion 108 of the silicon steel plate 102 because of the thermal stress 109 in the circumferential direction.
If the laminated core 101 is arranged in a bare state, the above-mentioned problem occurs in the ignition coil 100. To prevent this, the laminated core 101 is wound with the tape 103 as described above. In other words, as the tape 103 binds the laminated core 101 from the outer circumferential side, the laminated core 101 is prevented from being deformed elliptically. Moreover, as the tape 103 covers the silicon steel plate 102, the pointed corner portion 108 is enclosed. In this manner, the tape 103 relaxes the thermal stress applied to the epoxy resin 107a interposed in the gap 105.
The thickness of the tape 103 is in proportion to the quantity of thermal stress relaxation required of the tape 103. To be specific, the greater the thickness of the tape 103, the more the elliptic deformation of the laminated core 101 is suppressed. Because of this, the quantity of thermal stress relaxation is increased. Moreover, the greater the thickness of the tape 103, the more unlikely that roughness due to the corner portion 108 appears on the outer circumferential side of the tape 103. Therefore, the corner portion 108 is more unlikely to become the starting point of a crack.
Conventionally, however, there was no information about optimization of the thickness of the tape 103, namely a thermal stress relaxing member. Therefore, the life span of the epoxy resin 107a varied between an ignition coil having the tape 103 with a great thickness and one having the tape 103 with a small thickness because of defects such as a crack caused by a thermal stress.